It isn't quite so bad for most of the day, at least to Ely, because there are the GA and XC services too, filling in some of the connection gaps. But by the evening they've thinned out (GA) or stopped (XC) and so you hit these large gaps.
I was going to go to London tomorrow (well, today now) but as doing so will now involve leaving earlier, sitting around at Cambridge for almost an hour, getting home an hour later than planned, and also having to sit on a Thameslink 700 for about two and a half hours, doing so now looks considerably less attractive. I'll probably cancel my plans and spend my money on something else.
The other problem is that it appears that nothing later than usual is planned to leave Cambridge going north, so to get home to Ely or Kings Lynn will involve leaving London about 35 minutes earlier than the usual last train at 2312 in order to get the last connection from Cambridge. Not a lot of use if you've got theatre tickets or similar.
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Edit : well, I'm currently at Cambridge, and not only are the fasts cancelled as we already know, but so are the 1424 Brighton and the 1427 stopper. So the next London GN/TL train is the 1454 Brighton, assuming they can work out how to run that. One train in an hour when there should be five! Rather glad I cancelled my plans to go to London now.